The present invention relates generally to ultrasonic imaging and more particularly relates to the use of harmonic imaging to guide and monitor the application of therapeutic ultrasound.
It is known in the art of medical imaging and therapy that ultrasonic energy can be used for both diagnostic purposes and therapeutic purposes. For example, high-intensity focused ultrasound (HIFU) beams can be used to treat tumors by causing local focal temperature increases that cause cell necrosis. In using HIFU, the location of the focused ultrasound beam must be determined to place the beams focal point on the tissue (tumor) which is targeted for therapy. In addition, it is desirable to sense and monitor changes which are induced by the HIFU beam within the exposed tissue.
It is known that non-linear propagation occurs in a medium, such as tissue, which is exposed to intense ultrasound pressure, such as that which occurs from a HIFU beam. The HIFU beam is a high intensity pressure wave which alternately compresses and relaxes the tissue during a signal cycle. As a beam propagates, regions of compression can disturb local propagation speeds and result in regions of increased speeds in compression segments and decreased propagation speeds in rarefaction segments of the wave. This effect locally increases with increasing peak pressure values and also exhibits a cumulative nature, i.e., becoming more prominent as an intense beam propagates further into a medium. This tends to distort the propagating pressure wave and enhance non-linearities in the echo signal. This results in a generation of higher-order harmonics and mixing products in a propagating ultrasound signal. This process is altered, however, by attenuation losses in tissue, which typically increase with increasing frequency.
The use of harmonic imaging in diagnostic imaging is also known in the prior art. However, such systems have been generally used to image non-linear scattering from small, gas-filled contrast agent particles. An example of this can be found in applicants"" copending application, Ser. No. 09/318,882, filed on May 26, 1999 and entitled, xe2x80x9cUltrasonic Systems and Methods for Fluid Perfusion and Flow Rate Measurement,xe2x80x9d which is hereby incorporated by reference. There are also reports that different tissues exhibit different non-linear properties that can be observed in second-harmonic images if the peak pressures of the launched pulse are sufficiently large.
Although harmonic imaging techniques have been evaluated and the non-linear properties of tissues have been observed in the past, these effects have not been used in an advantageous matter to guide and monitor the progress of therapeutic ultrasound.
It is an object of the present invention to provide a system and method for aiming a therapy ultrasound beam.
It is a further object of the present invention to provide a system and method for monitoring the application of a therapy ultrasound beam using harmonic imaging.
It is another object of the present invention to provide a system and method for detecting thermal lesions resulting from the application of therapeutic ultrasound and determining the position of such lesions using ultrasound harmonic imaging.
A present method is used for aiming or directing a focused ultrasonic therapy beam. The method begins by acquiring a first image scan using a first frequency ultrasound signal. A second image scan is then acquired using the first frequency ultrasound signal in the presence of a therapy beam ultrasound signal. Difference properties in the first and second image scans are then identified to determine where the focused ultrasonic therapy beam is currently directed. The difference properties can be obtained by generating a difference image from the first image and second image scans. The difference image can then be superimposed on the first image scan and displayed to illustrate the presence of the focused ultrasonic therapy beam.
Preferably, the method described is used in conjunction with an ultrasound system which includes a diagnostic ultrasound transducer and a high intensity focused ultrasound therapy transducer which are arranged in a collinear fashion. It is also preferred that the image scans take the form of non-linearity imaging scans, such as harmonic imaging or pulse inversion imaging.
A further method is provided for operating an ultrasound therapy system having a therapy ultrasound transducer and a diagnostic ultrasound transducer to direct the application of the therapy ultrasound. The method includes operating the diagnostic ultrasound transducer for a first interval to acquire a first ultrasound image scan; simultaneously operating the diagnostic ultrasound transducer and therapy ultrasound transducer for a second interval to acquire a second ultrasound image scan; and determining a difference in the first and second image scans indicative of the pattern of the therapy ultrasound transducer signal.
A method of monitoring ultrasound therapy is also provided. This method begins by acquiring a first ultrasound image (baseline) of a region subjected to therapy. High intensity ultrasound is then applied to the region for a first time period. A second ultrasound image of the region is then acquired after the first time period. An aggregate induced effect can be determined based on the first and second image scans. The high intensity ultrasound can then be discontinued for a second time period and a third ultrasound image is then acquired. Transient changes due to in-situ heating in the region and permanent changes due to alteration in tissue microstructure in the region can then be determined.
Generally, the first time period is selected to be long enough such that the applied high intensity ultrasound has a therapeutic effect. The second time period is selected to allow cooling of the region undergoing high intensity ultrasound therapy.